NO329355B1 - Procedure for a Fischer-Tropsch reaction - Google Patents

Abstract

The present invention includes Fischer-Tropsch catalysts, reactions using Fischer-Tropsch catalysts, methods of making Fischer-Tropsch catalysts, processes of hydrogenating carbon monoxide, and fuels made using these processes. The invention provides the ability to hydrogenate carbon monoxide with low contact times, good conversion rates and low methane selectivities. In a preferred method, the catalyst is made using a metal foam support.

Description

Field of invention.

The present invention relates to a method for a Fischer-Tropsch reaction.

Background for the invention.

Fischer-Tropsch synthesis is carbon monoxide hydrogenation usually performed on a product stream from another reaction, which includes but is not limited to steam reforming (product stream H2/CO ~ 3), partial oxidation (product stream H2/CO ~ 2) , autothermal reforming (product stream H2/CO ~ 2.5), C02 reforming (H2/CO ~ 1), coal gasification (product stream H2/CO ~ 1) and combinations of these.

Fundamentally, Fischer–Tropsch synthesis has fast surface reaction kinetics. But the overall reaction rate is severely limited by heat and mass transfer with conventional catalysts or catalyst structures. The limited heat transfer together with the fast surface reaction kinetics can result in hot spots in a catalyst bed. Hot spots favor methanation. In commercial processes, fixed-bed reactors with small internal diameters or slurry-type and fluidized-type reactors with small catalyst particles (>50 ym) are used to mitigate heat and mass transfer limitations. In addition, one of the important reasons why Fischer-Tropsch reactors are operated with low conversions per passage to minimize temperature fluctuations in the catalyst bed. Due to the required operating parameters to avoid methanation, conventional reactors are not improved, even with more active Fischer-Tropsch synthesis catalysts. Detailed operation is summarized in table 1 and fig. 1.

For references that contained results for several test conditions, the test that best responded to the conversion, selectivity and/or conditions according to the invention was selected for comparison of contract time.

(A) References:

1. S. Bessell, Appl. Catal. A: Gen., Vol 96, 1993, p 253.

2. G.J. Hutchings, Topics Catal., Vol 2, 1995, p 163.

3. US Patent 4,738,948

4. E. Iglesia, S.C. Reyes, R.J. Madon and S.L. Soled, Adv.Catal.,

Vol 39, 1993, p 221. 5. F.S. Karn, J.F. Shultz and R.B. Anderson, Ind. Meadow. Chem. Prod Res.

Dev., Vol 4(4), 1965, p 265.

6. F. King, E. Shutt and A.I. Thomson, Platinium Metals Rev.,

Vol 29(44), 1985, p 146.

7. J.F. Shultz, F.S. Karn and R.B. Anderson, Rep. Invest. -

U.S. Cage. Mines, Vol 6974, 1967, p 20.

Literature data (Table 1 and Fig. 1) were obtained at lower H2/CO ratio (2:1) and longer contact time (3 sec or more) in a fixed bed type reactor. Low H2/CO (especially 2-2.5), long contact time, low temperature and higher pressure favor Fischer-Tropsch synthesis. Selectivity for CH4 is significantly increased by increasing the H2/CO ratio from 2 to 3. Increasing contact time also has a dramatically beneficial effect on catalyst performance. Although reference 3 in table 1 shows satisfactory results, the experiment was carried out under the conditions where Fischer-Tropsch synthesis is favored (at least 3 sec. residence time and H2/CO=2). Additionally, the experiment in reference 3 was conducted using a powdered catalyst on a pilot scale that would be impractical commercially due to the pressure drop caused by the powdered catalyst. Operation at a higher temperature will increase the conversion, but at a much higher cost to the selectivity for CH4. It is also worth mentioning that contact time in commercial Fischer-Tropsch units is at least 10 sec.

European patent application 83200208.3 is considered the closest prior art.

Consequently, there is a need for a catalyst structure and a method for Fischer-Tropsch synthesis which can achieve the same or higher conversion with shorter contact time and/or with higher H2/CO.

Summary of the invention.

Thus, the method according to the present invention is characterized in that it comprises the steps: (a) providing a catalyst structure having a first porous structure with a first pore surface area and a first pore size of at least 0.1 µm,

a porous intermediate layer with a second pore surface area and a second pore size that is smaller than said first pore size, where the porous intermediate layer is placed on said first pore surface area, and

a Fischer-Tropsch catalyst selected from the group consisting of cobalt, ruthenium, iron, rhenium, osmium and combinations thereof located on said second pore surface area, and

(b) passing a feed stream of a mixture of hydrogen gas and carbon monoxide gas through

the catalyst structure, and heating of

the catalyst structure to at least 200°C at operating pressure, where the supply stream has a residence time i

the catalyst structure in less than 5 seconds, whereby a product vapor is obtained with at least 25% conversion of carbon monoxide, and at most 25% selectivity for methane.

It is preferred that the method is carried out in a reaction chamber in which the catalyst has a thickness of 1.5 cm or less, and touches or is close to a reaction chamber wall.

It is preferred that the method includes the production of hydrocarbons so that: at least 1 ml per minute of product in liquid form is produced, where the volume of the liquid product is measured at 20<*>0 and 1 atmosphere, or

at least 1 1 per minute of hydrocarbon product in gaseous form is produced, where the hydrocarbon product contains at least 2 carbon atoms per molecule, and the volume of gaseous product is measured at 20<<*>C and 1 atmosphere.

It is preferred that the supply current has a contact time of less than about 2 seconds, more preferably 0.1 to 1 second.

It is preferred that the feed stream has a steam-to-carbon ratio of 2:1 to 3.5:1.

It is preferred that the catalyst has a thickness of about 1 mm to 10 mm, more preferably 1 mm to 3 mm.

It is preferred that a reaction chamber wall separates the reaction chamber from a cooling chamber.

It is preferred that the selectivity to methane is less than 20%, and the carbon monoxide conversion is at least 50%, and more preferably that the selectivity to methane is between 15% and 5%.

It is preferred that the pressure loss through the reaction chamber is less than 0.7 atm (10 psig).

It is preferred that microchannels are provided on at least one reaction chamber wall on the opposite side of the reaction chamber from the catalyst structure.

It is preferred that the catalyst has a surface area of more than 0.5 m<2>/g when measured by BET, and more preferably more than 2.0 m<2>/g.

Advantages that can be achieved with the present invention include (i) with residence/contact times that are shorter than according to known techniques, higher conversions are achieved without an increase in methane selectivity, and (ii) when residence/contact times increase, conversion increases and methane selectivity decreases. It has surprisingly been shown that carbon monoxide can be hydrogenated with a short contact time to form liquid propellants at good conversion levels, low methane selectivities and good production rates.

Brief description of the drawings.

Fig. 1 shows a graph of CO conversion versus contact time for known Fischer-Tropsch processes. Fig. 2 shows a cross-section of a catalyst structure according to the invention. Fig. 3 shows a reactor design which has several reaction chambers, each of which contains a catalyst, as well as several heat exchangers.

Description of preferred/preferred embodiment(s).

A catalyst shown in fig. 2 has a porous carrier 100, a buffer layer 102, a boundary layer 104 and, optionally, a catalyst layer 106. Each layer can be continuous or discontinuous in the form of points or dots, or in the form of layers with slits or holes.

The porous carrier 100 may be a porous ceramic material or a porous metal. Porous supports which are suitable for use in the invention include carbides, nitrides and composite materials. Before deposition of the layers, the porous support preferably has a porosity of from approx. 30% to approx. 99%, more preferably from 60% to 98%, as measured by mercury porosimetry, and an average pore size of from 1 µm to 1000 µm as measured by optical and scanning electron microscopy. Preferred forms of porous supports are foam, felt, wadding and combinations thereof. Foam is a structure with continuous walls that define pores throughout the structure. Felt is a structure of fibers with cavities between them. Cotton wool is a structure of tangled threads, like steel wool. Less preferably, porous carriers can also comprise other porous media, such as pellets and honeycombs, provided they have the above-mentioned porosity and pore size characteristics. The open cells in a metal foam are preferably from approx. 8 pores per cm (ppcm) to approx. 1200 ppcm and more preferably from approx. 16 to approx. 240 ppm. PPCM is defined as the largest number of pores per cm (irrelevant in isotropic materials, but in anisotropic materials the measurement is carried out in the direction that maximizes the number of pores). According to the present invention, ppcm is measured by scanning electron microscopy. It has been observed that a porous carrier provides several advantages in the invention, such as low pressure drop, increased thermal conductivity compared to conventional ceramic pellet carriers, as well as easy loading/unloading in chemical reactors.

The buffer layer 102 has, if used, a different composition and/or density than both the carrier and the boundary layer. Preferably, the buffer layer is a metal oxide or metal carbide. In connection with the invention, it has been shown that vapor-deposited layers are particularly good because they exhibit better adhesion and resist flaking, even after several heating cycles. Preferably, the buffer layer is Al2O3, TiC>2, SiC>2, and ZrC>2 or combinations thereof. More specifically, AI2O3, (XAI2O3, Y-AI2<O>3 and combinations thereof. a-Al2C>3 is more preferred due to its excellent resistance to oxygen diffusion. Therefore, it is expected that the resistance to high temperature oxidation can be improved with aluminum oxide coated on the porous support 100. The buffer layer can also be formed from two or more sub-layers with different compositions. When the porous support 100 is metal, for example a stainless steel foam, a preferred embodiment has a buffer layer 102 formed from two sub-layers (not shown) of different composition. The first sub-layer (in contact with the porous support 100) is preferably TiC>2 because it exhibits good adhesion to the porous metal support 100. The second sub-layer is preferably a-Al 2 O 3 placed on the layer of TiC>2. It is preferred that the a-Al2O3 sublayer is a dense layer that provides excellent protection of the underlying metal surface. A less dense boundary layer of aluminum oxide with a large surface area can then be deposited which supports a catalytically active layer.

Typically, the porous burr 100 has a coefficient of thermal expansion that is different from the coefficient of thermal expansion of the boundary layer 104. Consequently, for high-temperature catalysis (T>150<<>,C) a buffer layer 102 can be used as a transition between two thermal expansion coefficients. The coefficient of thermal expansion of the buffer layer can be tailored by adjusting the composition to achieve a coefficient of thermal expansion that is compatible with the coefficients of thermal expansion of the porous carrier and the boundary layers. Another advantage of the buffer layer 102 is that it provides resistance to side reactions, such as coking or cracking caused by a bare metal foam surface. For chemical reactions that do not require high surface area supports, such as catalytic combustion, the buffer layer 102 stabilizes the catalyst metal as a result of strong metal-metal-oxide interaction. In chemical reactions that require carriers with a large surface area, the buffer layer 102 provides stronger binding to the boundary layer 104 with a large surface area. Preferably, the buffer layer is without openings and pinholes. This ensures very good protection of the underlying carrier. More preferably, the buffer layer is non-porous. The buffer layer has a thickness that is less than half the average pore size of the porous carrier. Preferably, the buffer layer is between approx. 0.05 and approx. 10 <y>m thick, more preferably less than 5 <y>m thick. The buffer layer should exhibit thermal and chemical stability at higher temperatures.

In some cases, sufficient adhesion and chemical stability can be achieved without a buffer layer, so that the buffer layer can be omitted and thereby save cost, provide extra volume and further increase the heat transport from the catalyst.

The boundary layer 104 can be constituted by nitrides, carbides, sulphides, halides, metal oxides, carbon and combinations thereof. The boundary layer provides a large surface area and/or ensures a desirable catalyst-carrier interaction for catalysts on a carrier. The boundary layer can comprise any material that is used conventionally as a catalyst carrier. Preferably, the boundary layer is a metal oxide. Examples of metal oxide include, but are not limited to, Y-AI2O3, SiC<>>2, ZrC>2, Ti02' tungsten oxide, magnesium oxide, vanadium oxide, chromium oxide, manganese oxide, iron oxide, nickel oxide, cobalt oxide, copper oxide, zinc oxide, molybdenum oxide, tin oxide, calcium oxide, aluminum oxide, oxide(s) of the lathan series, zeolite(s) and combinations thereof. The boundary layer 104 can function as a catalytically active layer without any further catalytically active material being deposited on it. Usually, however, the boundary layer 104 is used in combination with a catalytically active layer 106. The boundary layer can also be formed from two or more sub-layers of different composition. The boundary layer has a thickness that is less than half the average pore size of the porous support. Preferably, the boundary layer thickness is from approx. 0.5 to approx. 100 ym, more preferably from approx. 1 to approx. 50 etc. The boundary layer can be either crystalline or amorphous and preferably has a BET surface area of at least 1 m^/g.

The catalytically active material 106 (when used) can be deposited on the boundary layer 104. Alternatively, a catalytically active material can be deposited simultaneously with the boundary layer. The catalytically active layer (when used) is typically intimately distributed on the interface layer. That the catalytically active layer is "placed on" or "deposited on" the boundary layer includes the common understanding that microscopic, catalytically active particles are distributed: on the surface of the carrier layer (i.e. the boundary layer), in cracks in the carrier layer and in open pores in the carrier layer. Here, a Fischer-Tropsch catalytic metal is used in the catalytically active layer. Conventional Fischer-Tropsch catalysts are based on iron (Fe), cobalt (Co), nickel (Ni), ruthenium (Ru), rhenium (Re), osmium (Os) and combinations thereof. Catalytic metals are preferably iron, cobalt, ruthenium, rhenium, osmium and combinations thereof. In addition to these catalyst metals, a promoter can be added. Promoters may include transition metals and metal oxides (with the exception of Au and Hg), lanthanide metals or metal oxides and elements of group IA (with the exception of H). A Fischer-Tropsch catalytic metal combined with a suitable support, such as the interfacial porous support described herein, is termed a supported Fischer-Tropsch catalytic metal. Less preferred is the Fisch-Tropsch catalytic metal on support or a Fischer-Tropsch catalytic metal supported on other supports, such as a powder.

In order to mitigate the mass transfer limitation in the catalyst structure, the catalyst impregnation preferably forms a porous boundary layer having a depth of less than 50 µm, preferably less than 20 µm. Therefore, the diffusion path length is a factor of 5 shorter than for standard catalyst particles. The thinner, impregnated catalyst structure also increases heat transport as a result of a shorter heat transport path and leads to lower selectivity for CH4.

The catalyst structure can have an arbitrary geometric design. Preferably, the catalyst is a porous structure, such as a foam, a felt, a wadding and combinations thereof. The catalyst (comprising the support and Fischer-Tropsch catalytic metal) is preferably of such a size that it fits in a reaction chamber. The catalyst can be a single piece of porous, cohesive material, or many pieces in physical contact. The catalyst is preferred to have continuous material and continuous porosity, so that the molecules can diffuse through the catalyst. The catalyst can preferably be placed in a reaction chamber so that gases will flow mostly through the catalyst (a single piece or several pieces) instead of around it. It is preferred that the cross-sectional area of the catalyst occupies at least 80%, more preferably at least 95%, of the cross-sectional area of the reaction chamber. It is preferred that the catalytically active metal is distributed on surfaces through the catalyst so that reactants passing through the catalyst can react anywhere along the passage through the catalyst. This is a significant advantage over pellet-type catalysts which have a large volume of unused space or catalytically inefficiently used space in the interior of the pellet. The porous catalyst is also better than the powder because packed powders can cause severe pressure drop. The catalyst preferably has a surface area, measured by BET, of more than approx. 0.5 m^/g, preferably more than approx. 2.0 m2/g.

In addition, because it is not necessary for the catalyst structure to be abrasion resistant, as the catalyst particles used in a fluidized bed reactor must be, greater porosity can be used, e.g. porosity of over approx. 30%, thereby increasing mass transfer in the catalyst structure.

Catalysts can also be characterized by the properties they exhibit. Factors that can be controlled to effect these properties include the choice of the porous support layer, buffer layer, boundary layer and catalytically active layer; grading of thermal expansion coefficients, crystallinity, metal-support interactions, catalyst size, thermal conductivity of the support, porosity, heat conduction from the reaction chamber, deposition techniques and other factors that will appear from the description here. Certain preferred embodiments of the catalyst exhibit one or more of the following properties: adhesiveness, after 3 heat cycles in air the catalyst exhibits less than 2% (area) flaking according to SEM (scanning electron microscope) analysis; oxidation resistance, carbon monoxide conversion, contact times, methane selectivity, pressure drop and production rates.

Oxidation resistance can be measured by thermal gravimetric analysis (TGA). After heating by SSCC in air for 2500 minutes, the weight of the catalyst increases less than 5%, preferably less than 3%. Even more preferably, after heating at 750°C in air for 1500 minutes, the catalyst increases in weight by less than 0.5%. Each thermal cycle consists of heating from room temperature to GOO^C in air at a heating rate of 10^C/min., holding the temperature at GOO^C for 3000 minutes and cooling at a rate of 10<*>C/min.

One aspect of the present invention is a method where the catalyst is used so that lower methane selectivity is achieved at lower pressure. It was unexpectedly observed that when using the porous catalyst structure according to the invention, lowering the pressure in the Fischer-Tropsch reaction resulted in increased yield and less selectivity towards methane. See example 2.

Increased heat transfer in the present invention enables shorter contact times, good conversion and low methane selectivities. Various factors that can be used to increase heat transfer include: use of a metal carrier, preferably a porous metal carrier, such as a metal foam or metal wool, a thin buffer (if used) and boundary layer, a heat exchanger in thermal contact

with the reaction chamber, microchannels in the reaction chamber and/

or heat exchanger, as well as a catalyst which has a thickness in the direction of heat transfer (e.g. the direction of the reaction chamber walls and largely perpendicular to the direction of flow) of approx. 1.5 cm or less, more preferably from approx. 1 to 10

mm and even more preferably from approx. 1 to 3 mm.

Apparatus (ie reactors) and methods for hydrogenating carbon monoxide are described herein. The catalytic process is preferably carried out in an apparatus equipped with microchannels. A microchannel has a characteristic dimension that is less than approx. 1 mm. According to one embodiment, the reaction chamber has walls that define at least one microchannel through which reactants pass into the reaction chamber. It is preferred that the reaction chamber walls separate the reaction chamber from at least one cooling chamber. Examples of suitable microchannel devices and various process-related factors are described in US patent documents 5,611,214, 5,811,062 and 5,534,328 as well as in US patent applications 08/883,643, 08/938,228, 09/123,779, 09/375,614 (expire date 08/17/99 ) as well

09/265.227 (run date 08.03.99). Optionally, the catalyst is a monolith, i.e. a single continuous, yet porous piece of catalyst made of several continuous pieces stacked together (not a layer of packed powder or pellets or a coating on the wall of a microchannel) that can be easily inserted and removed from a reaction chamber. The piece or stack of catalyst pieces preferably has a width of from 0.1 mm to approx. 2 cm with a preferred thickness of less than approx. 1.5 cm, more preferably less than approx. 1.0 cm and even more preferably from approx. 1 to approx. 3 mm. The catalyst can provide numerous advantages to catalytic processes, such as: chemical stability, stability in repeated thermal cycles, thermal stability, efficient loading and unloading of catalysts, high rates of heat transfer and mass transfer as well as retention of the desired catalytic activity.

The metal surfaces inside the microchannel device can be coated with either the buffer layer or the boundary layer or both. This can be carried out by using any one of the processes described here, preferably by vapor deposition. Preferred coating materials include titanium oxide and 5-10% SiO2/Al2C>3. The internal surfaces in the reaction ion chamber, the heat exchanger and other surfaces in the microchannel apparatus can be coated. In some cases, the walls of the reaction chamber can be coated with a possible buffer layer, a boundary layer and a catalytically active material. Typically, the catalytically active material and the boundary layer are combined to form a supported catalyst. Coatings can also be applied to metal walls in pipes and pipelines that form connections to or inside the microchannel apparatus.

According to a preferred method according to the invention, a residence time of less than 5 sec. is achieved by: (a) using a catalyst structure of a metal foam with a catalyst on it, and (b) passing a feed stream containing a mixture of hydrogen gas and carbon monoxide gas through the catalyst structure and heating the catalyst structure to at least 200<*>C to achieve a product stream with at least 25% conversion of carbon monoxide and no more than 25% selectivity towards methane. In another method, the catalyst structure comprises a buffer layer and a boundary layer with a catalytically active metal placed on the boundary layer.

Methods for the hydrogenation of carbon monoxide are shown here. It is preferred that the ratio between hydrogen and carbon monoxide is from approx. 1:1 to approx. 6:1, preferably from approx. 2:1 to approx. 3.5:1. Hydrogenation is preferably carried out at temperatures above approx. 200<*>0, more preferably between 200<*>C and approx. 300<*>C and even more preferably between approx. 200<*>0 and approx. 270<*>0.

Certain embodiments of the invention are characterized in terms of residence time or contact time. These designations have well-defined meanings in the area. Contact time is the catalyst chamber's total volume divided by the total flow rate of input reactants assumed to be an ideal gas corrected to standard conditions (ie catalyst chamber volume/F-total at STP, where STP is 273K and 1 atm). The volume of the catalyst chambers includes the volume in the immediate vicinity of and around the catalyst zone. As an example, if a quarter of the channels are to be packed with powder, the volume of the catalyst chamber will not include the area where gas can flow and where it can come into contact with the catalyst, i.e. only a quarter of the total channel volume will be included in this calculation . The volume of dead space, i.e. collection tanks, discharge tanks etc., is omitted in this calculation. The average residence time (also referred to as residence time) is the catalyst chambers' total volume divided by the total flow rate of the input reactants corrected to the reactants' current temperature and pressure in the reactor (i.e. the catalyst chamber's volume/F-total corrected to current conditions). F-total at STP is the total volumetric flow rate of reactants (includes all reactants, as well as any thinners). Inlet gases are typically measured with mass control regulators set to standard conditions, ie the user presets the desired STP flow rate. F-total corrected to current conditions = F-total-STPx(temperature in K)/273 x 1 atm/(P-current in atm). This value is used to calculate the residence time or "true time" inside a reactor. Most practitioners prefer to use contact time because it is a convenient method to hold the time variable fixed while steps of lO^C are carried out in reaction temperature etc.

Contact times of less than 5 seconds. can be achieved with standard equipment, but at the expense of significant energy to increase the reactant space velocity to increase the pressure drop and poorer heat transfer, leading to higher methane formation. Consequently, the method according to the invention is preferably carried out in a reaction chamber where the catalyst has a thickness of approx. 1.5 cm or less and touches or is close to (within about 1 mm) of a reaction chamber wall, where the reaction chamber wall is in thermal contact with a heat exchanger. Heat transfer from the reaction chamber is preferably increased by the addition of microchannels on at least one reaction wall on the opposite side of the reaction chamber in relation to the catalyst structure. The catalyst preferably has continuous and relatively large pores, such as in a foam, to avoid large pressure losses. Preferably, the pore size in the large pores in the catalyst is between approx.

10 ym and approx. 300 <y>m.

Carbon monoxide hydrogenation is carried out at a contact time of less than 5 sec., more preferably less than approx. 2 sec. and even more preferably between approx. 0.1 and approx. 1 sec. With these contact times, good CO conversion and low methane selectivity can be achieved. Preferably, CO conversion is at least 25%, more preferably at least 50% and even more preferably more than about 80%. Methane selectivity is preferably less than 25%, more preferably less than approx. 20% and even more preferably between approx. 15% and 5%. In addition, these properties can be achieved with low pressure losses through the reaction chamber. In the invention, pressure loss through the reaction chamber is preferably less than approx. 1 kg/cm<2>, more preferably less approx. 0.7 kg/cm<2> and even more preferably less approx. 0.4 kg/cm<2> and even more preferably less than approx. 0.07 kg/cm<2>. A method for producing the catalyst can include the steps of selecting a porous carrier 100, possibly depositing a buffer layer 102 on the porous carrier 100 and depositing a boundary layer over this. Optionally, a catalyst layer 106 can be deposited on the boundary layer 104, or both the boundary layer and the catalyst layer can be deposited simultaneously on the buffer layer 102.

Due to the fact that metal has net surfaces that are non-porous and smooth, deposition at a buffer layer or a boundary layer can be made difficult. One way to mitigate this problem is to roughen the metal surface by chemical etching. The adhesion of metal catalysts with a large surface area on a support, such as gamma aluminum oxide, to metal foam is significantly improved when metal foam is stirred up by chemical etching using mineral acid solutions, e.g. 0.1-1M HCl. The roughened net surface also exhibits better resistance to peeling of the catalyst layer during thermal cycles. In a preferred embodiment where a metal foam is used as the porous carrier 100, the metal foam is etched before vapor deposition of the buffer layer 102. Etching is preferably carried out with acid, e.g. HC1.

Deposition of the buffer layer 102 is preferably by vapor deposition which includes, but is not limited to, chemical vapor deposition, physical vapor deposition or combinations thereof. It has surprisingly been found that vapor deposition, which is typically carried out at high temperatures, results in polycrystalline or amorphous phases which provide good adhesion of the buffer layer to the surface of the porous carrier. This method is particularly advantageous for bonding a metal oxide buffer layer to a porous support of metal. Alternatively, the buffer layer 102 can be obtained by solution coating. E.g. solution coating has the steps of metal surface functionalization by exposure of the metal surface to water vapor to form surface hydroxyls, followed by surface reaction and hydrolysis of alkoxides to obtain a coating of metal oxide. This solution coating may be preferred as a reasonable method for depositing the buffer layer 102.

The boundary layer 104 is preferably formed by vapor or solution deposition using precursors, as is known for these techniques. Suitable precursors include organometallic compounds, halides, carbonyls, acetonates, acetates, metals, colloidal dispersions of metal oxides, nitrates, slurries, etc. E.g. a porous boundary layer of alumina can be wash-coated with a colloidal dispersion of PQ alumina (Nyacol Products, Ashland, MA) followed by drying in a vacuum oven overnight and calcination at 500<*>0 for 2 hours.

The catalytically active material can be deposited by any suitable method. E.g. catalytic metal precursors can be deposited on colloidal metal oxide particles and the slurry coating on a porous support and then dried and reduced.

Example 1.

The effect of residence time and reaction temperature on the catalytic conversion of CO with H2 was investigated in a constant flow reactor. The reactor was supplied with a mixture of feed gas consisting of H2 and CO in a molar or volumetric (assuming behavior as an ideal gas) ratio H2/CO=3. This reactant feed was fed into a reaction chamber maintained at a constant temperature inside an isothermal oven. The interior of the catalyst chamber measured 35.6 mm in length, 1.5 mm in thickness and 8 mm in width. The reaction products then left the reaction chamber and were collected and analyzed for composition.

The catalyst in this experiment was prepared as follows. First, acidic gamma alumina carrier powder (Strem) was ground and sieved to between 70 and 100 mesh (150-220 ym), and calcined (stabilized) at SOCC for several hours. This powder was then impregnated with a solution containing precursors of cobalt nitrate hexahydrate and ruthenium trichloride hydrate (or ruthenium nitrosyl nitrate), which were present in desired concentrations to form a catalyst of 15 wt% cobalt and 1 wt% ruthenium on aluminum oxide . The precursor solution was prepared in such a way that the pore volume in the aluminum oxide carrier was saturated without supersaturation of the aluminum oxide carrier. This powder was then dried in a vacuum oven at 10°C for at least 4 hours followed by drying at 100°C for at least 12 hours. The powder was then calcined by heating at 340<<*>C for at least 3 hours. A portion of the powder was then mixed with distilled water at a weight ratio of water to catalyst of at least 2.5 to prepare a catalyst slurry. This catalyst slurry was then placed in a container with inert grinding medium beads and placed in a rotary device for at least 24 hours. This slurry was then ready for coating a pretreated support of monolith-type metal foam. The metal foam carrier monolith is typically stainless steel with 80 ppi (pores per inch) (about 32 pores per cm) supplied by AstroMet, Cincinnati, Ohio) with characteristic macropores ranging in size from about 200 to approx. 250 ym and with a porosity of approx. 90% by volume. The monolith pretreatment consisted of cleaning in turn in dichloromethane and acetone solvents in a water bath immersed in an ultrasonic apparatus to shake the solvent together with the monolith. Optionally, the metal surface of the monolith can then be roughened up by etching with acid. If this is desired, the monolith is immersed in 0.1 molar nitric acid and placed in an ultrasound device. The monolith was then washed in distilled water and dried at approx. 100°C. The monolith was then coated with a layer of aluminum oxide using a chemical vapor deposition (CVD) technique. The CVD system had a horizontal hot wall reactor with three precursor sources. The CVD coatings were applied at a deposition temperature of 600<*>C and a reactor pressure of 5 torr. Aluminum iso-propoxide was used as the aluminum precursor. This precursor was stored in a quartz vessel which was maintained at 100<*>0 during deposition, forming a vapor which was fed into the CVD reactor with a stream of nitrogen carrier gas for approx. 20 minutes. Air was then used to oxidize the aluminum precursor to aluminum oxide. Typical thickness of the aluminum oxide coatings is approx. 0.5 ym. This pretreated metal support foam monolith was then coated with the catalyst slurry by dip coating. The monolith was then dried in flowing air or nitrogen at room temperature while the monolith was continuously rotated in such a way as to form an even coating of the dried catalyst slurry layer. The monolith was then dried at 90<*>0 for at least 1 hour, slowly heated to 120<*>C over at least 1 hour, further dried at 120<*>C for at least 2 hours and then heated to 350<*> 0 and calcined for at least 3 hours. Typically, 0.1-0.25 g of Co-Ru powder catalyst on aluminum oxide support was coated on the metal foam monolith with the above dimensions and characteristics.

The catalyst monolith or powder, which weighed approx. 0.5 g, was then placed inside the reaction chamber and activated (or reduced) before reaction by heating to from approx. 350<*>C to 400<*>C under a hydrogen-containing stream of from approx. 10 to 20% (by volume or mole) hydrogen in an inert carrier gas (such as nitrogen or helium) at a flow rate of at least 20 cm^/min. (measured at 273K and 1 atm) for at least 2 hours. The catalyst was then allowed to cool to reaction temperatures, at least 200°C. The catalyst was then exposed to a feed gas consisting of H2 and CO in a desired molar ratio of H2 per moles of CO. The flow rate of the feed gas can be regulated to achieve exactly a desired contact time, usually approx. 1 sec. The reaction products were then analyzed to evaluate the conversion of CO and the selectivity towards certain products, such as methane. The reaction was carried out at pressures of up to 24 atmospheres (approx. 25 kg/cm<2>).

Table El-1 shows the results of these experiments. In general, the powder form of the catalyst produced greater conversions at a given temperature than the monolith form. However, at a given temperature, the monolith catalyst formed less methane. In conventional Fischer-Tropsch reactors, methane formation is mainly affected by reactor temperature and the composition of the feed, although it is also affected to a lesser extent by other parameters, such as contact time. The fact that the monolith catalyst gives lower methane selectivity at a given temperature suggests that the monolith is better able to conduct heat away from the inner part of the reactor and thereby avoid higher local temperatures, which often exist in the inner parts of packed or powdered beds. For the monolith catalyst, conversion is a strong function of both temperature and contact time, and conversion will increase with increasing temperature and/or time. Lowering the contact time from 2 sec. to 1 sec. at 275<<>>C for the monolith catalysts resulted in lower conversion and higher methane selectivity.

Compared to the results of previous investigations in table 1, several characteristics are clear: - compared to all these references, sufficient catalyst performance (conversion of over approx. 50% and methane selectivity below approx. 25%) can be achieved at a contact time that is from approx. three to twelve times shorter. - formation of methane, which is strongly promoted by high reactor temperatures and the ratio between hydrogen and carbon 1 feed, lies between references 1 and 3 which use the most similar contact times, but the monolith catalyst gives comparable methane selectivities under conditions that are much more unfavorable than those used in these references. The monolith mold was able to form this amount of methane at temperatures up to 260°C (compared to 240°C in reference 1) and a H2 to CO feed ratio of 3 (compared to 2 for references 1 and 3). This further shows that the monolith form removes heat more efficiently than the powder or pellet forms and that methane formation can be suppressed, even under undesirable conditions. - at a comparable 1^:00 feed ratio of 3 and comparable CO conversion (approx. 80%), the powdered catalyst in reference 7 gives much higher selectivity for methane than the catalyst according to the invention, even at lower temperatures and longer contact times where methane formation is unfavorable . Note that in reference 7, methane selectivity was nearly tripled by changing the 1^:00 feed ratio from 2 to 3.

In addition, the thickness of the catalyst layer in the monolith (typically less than 20 ym) is much smaller than the finest particle size used either in fixed-bed reactors (over 100 ym) or slurry-type or fluidized-type reactors (over 50 ym). Therefore, the internal mass transport diffusion path is shorter in the monolith catalyst. Moreover, in Fischer-Tropsch synthesis operations, internal pores in the catalyst are normally filled with hydrocarbon products, and hydrogen has a much higher conductivity than CO. This could result in a much higher H2/CO ratio inside a pellet or powder catalyst than in the mass feed, which promotes methane formation. Therefore, the thinner catalyst layer on the monolith catalyst will result in a relatively lower local P^ concentration inside the catalyst to minimize selectivity for methane. Yet another advantage of porous catalysts is their efficient novel reversal in which molecules can pass through and react within the catalyst without causing excessive pressure loss.

Example 2.

An experiment was carried out to show operation at different pressures. The equipment was the same as in example 1.

According to the literature, the variation in pressure should only affect the true residence time in Fischer-Tropsch synthesis. In other words, conventional wisdom in Fischer-Tropsch reactions is that the reaction rate is proportional to pressure at the same residence time. However, as shown in table E2-1, with the catalyst structure according to the invention, the activity of the catalyst was unexpectedly increased when the pressure was lowered at the same residence time. This surprising result is attributed to the increased mass and heat transfer that is possible with the catalyst structure according to the invention.

Example 3.

Application of Co or Ru alone on a carrier of acidic gamma-alumina as a catalyst on the metal foam was also tested under the conditions of example 1, and the behavior was found to be worse than for bimetallic catalyst, such as Co-Ru.

Example 4.

An experiment was carried out to show certain advantages of the buffer layer according to the invention.

An unetched stainless steel foam (Astromet, Cincinnati, OH) was coated with 1000 Å TiO 2 by chemical vapor deposition. Titanium isopropoxide (Strem Chemical, Newburyport, MA) was vapor deposited at a temperature of from 250 to 280<*>0 at a pressure of from 0.1 to 100 torr. Titanium oxide coating with excellent adhesion to the foam was obtained at a deposition temperature of 600°C and a reactor pressure of 3 torr.

SEM (scanning electron microscope) analysis showed that the gamma aluminum oxide on the stainless steel support and with a Ti02 ~ buffer layer did not show peeling after several (3) thermal cycles from room temperature to 600<*>C. In a control experiment with a support of stainless steel foam coated with gamma aluminum oxide without a TiC^ buffer layer, strong flaking or peeling of the gamma aluminum oxide was observed under the same test conditions. The uncoated steel foam oxidized rapidly when heated to 500°C in air (as shown by values for thermal weight gain, i.e. thermal gravity), while the titanium dioxide-coated steel oxidized relatively slowly. Similarly, uncoated nickel foam oxidized, while under the same conditions (heating to 500°C or 750°C in air) titanium dioxide-coated nickel foam showed zero (ie, undetectable levels of) oxidation.

Claims (15)

1. Method for a Fischer-Tropsch reaction, characterized in that it comprises the steps: (a) providing a catalyst structure having a first porous structure with a first pore surface area and a first pore size of at least 0.1 µm, a porous intermediate layer with a second pore surface area and a second pore size that is smaller than said first pore size, where the porous intermediate layer is placed on said first pore surface area, and a Fischer-Tropsch catalyst selected from the group consisting of cobalt, ruthenium, iron, rhenium, osmium and combinations thereof located on said second pore surface area, and (b) passing a feed stream of a mixture of hydrogen gas and carbon monoxide gas through the catalyst structure, and heating the the catalyst structure to at least 200°C at operating pressure, where the feed stream has a residence time in the catalyst structure of less than 5 seconds, whereby a product vapor is obtained with at least 25% conversion of carbon monoxide, and at most 25% selectivity for methane. 2. Method according to claim 1, carried out in a reaction chamber in which the catalyst has a thickness of 1.5 cm or less, and touches or is close to a reaction chamber wall. 3. Method in accordance with one or more of claims 1 and 2, which includes the production of hydrocarbons so that: at least 1 ml per minute of product in liquid form is produced, where the volume of the liquid product is measured at 20°C and 1 atmosphere, or at least 1 1 per minute of hydrocarbon product in gaseous form is produced, where the hydrocarbon product contains at least 2 carbon atoms per molecule, and the volume of gaseous product is measured at 20^ and 1 atmosphere. 4. Method in accordance with one or more of claims 1 - 3, where the supply current has a contact time of less than about 2 seconds. 5. Method in accordance with one or more of claims 1 - 4, where the supply current has a contact time of from 0.1 to 1 second. 6. Method according to one or more of claims 1 - 5, wherein the feed stream has a steam-to-carbon ratio of 2:1 to 3.5:1. 7. Method according to one or more of claims 1 - 6, where the catalyst has a thickness of approximately 1 mm to 10 mm. 8. Method according to one or more of claims 1 - 6, where the catalyst has a thickness of approximately 1 mm to 3 mm. 9. Method according to one or more of claims 1-8, where a reaction chamber wall separates the reaction chamber from a cooling chamber. 10. Method according to one or more of claims 1 - 9, where the selectivity to methane is less than 20%, and the carbon monoxide conversion is at least 50%. 11. Method in accordance with one or more of claims 1 - 10, where the selectivity to methane is between 15% and 5%. 12. Method according to one or more of claims 1 - 11, wherein the pressure drop through the reaction chamber is less than 0.7 atm (10 psig). 13. Method according to one or more of claims 1 - 12, where microchannels are provided on at least one reaction chamber wall on the opposite side of the reaction chamber from the catalyst structure. 14. Method according to one or more of claims 1 - 13, where the catalyst has a surface area of more than 0.5 m<2>/g when measured by BET. 15. Method according to one or more of claims 1 - 13, where the catalyst has a surface area of more than 2.0 m<2>/g when measured by BET.